batman-adv: Disallow zero and mcast src address for mgmt frames

The routing check for management frames is validating the source mac
address in the outer ethernet header. It rejects every source mac address
which is a broadcast address. But it also has to reject the zero-mac
address and multicast mac addresses.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>